1. Field of the Invention
This invention relates to an improvement in a vehicle final reduction gear unit equipped with friction brake for braking a vehicle.
2. Description of Background Art
A final reduction gear unit of an ATV (All Terrain Vehicle) equipped with a differential mechanism and multi-disc wet brake is disclosed, for example, in JP-A No. 2000-52943 (FIG. 2, FIG. 3).
As shown in FIG. 2 in JP-A No. 2000-52943, in an ATV, a prime mover (8) (a parenthesized number indicates a reference sign described in JP-A No. 2000-52943, the same shall apply hereinafter) is disposed in the center of the vehicle. The drive force generated by the prime mover (8) is transmitted through a drive shaft (12) to a differential mechanism (17). The drive force transmitted to the differential mechanism (17) is then transmitted to the right and left axles (20) to rotate the front wheels (1).
Further, as shown in FIG. 3 in JP-A No. 2000-52943, the differential mechanism (17) is housed in a differential case (18) which is a rotating body. The differential case (18) is housed in a reduction gear case (14) which is a stationary body. The drive force transmitted to the drive shaft (12) is transmitted from a small reduction gear (15) coupled to the drive shaft (12), to a large reduction gear (16) provided integrally with the differential case (18), thus rotating the differential case (18) and also the right and left axles (20).
The differential mechanism (17) includes a support shaft (32) secured to the differential case (18) at right angles to the axle (20), a plurality of small differential gears (31) rotatably mounted on the support shaft (32), and right and left large differential gears (30) meshing with the small differential gears (31) and respectively rotating integrally with the right and left axles (20). The rpm of the right and left axles (20) is appropriately distributed, thus providing a smooth vehicle driving.
The multi-disc wet brake (6) braking the differential case (18) occupies an area between the reduction gear case (14) and the differential case (18). In the multi-disc wet brake (6), a plurality of stationary friction discs (41) are spline-fitted into the inner peripheral surface of the reduction gear case (14) and are able to move in the direction of the axle and are alternated in position with a plurality of rotative friction discs (40) which are spline-fitted into the outer peripheral surface of the differential case (18) and are able to move the direction of the axle. A braking disc (42) is operated to apply a pressing force in the axial direction to generate a frictional force, thus braking the differential case (18).
A friction device (35) includes a plurality of side friction discs (36b) which are spline-fitted into the inner peripheral surface of the differential case (18) and are able to move in the support shaft, a plurality of axle-side friction discs (36a) which are spline-fitted into the left large differential gear (30) and are able to move in the axial direction, and a coned disc spring (37) which press the friction discs (36a, 36b) toward the axial direction. A differential between the right and left axles (20) can be limited to a certain extent by the friction device (35).
It may be desired to lock the differential of an ATV between the right and left axles (20). However, if a diff-lock operation device for locking a differential is mounted on the friction device (35), the installation area of the diff-lock operation device is additionally required, leading to an increased size of the final reduction gear unit. Under these circumstances, a desired final reduction gear unit can be designed to be compact in size even when it is equipped with a diff-lock operation device.